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Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System's planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers. Only recently have space missions begun to solve this beguiling Solar System mystery--that a small number of distant moons have been successfully hiding, from the curious eyes of astronomers, life-sustaining liquid water beneath secretive shells of ice. "We are just beginning to try and figure out quantitatively how all this might smooth a surface," Dr. Thomas said in the May 17, 2013 New Scientist.